GaN-based compound semiconductor materials have recently attracted attention as semiconductor materials for short wavelength light-emitting devices. GaN-based compound semiconductors are formed on substrates composed of sapphire single crystals, various other oxides, or III-V compounds by a method such as metalloorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
One of the characteristics of GaN-based compound semiconductor materials is having low current diffusion in the horizontal direction. Although this is thought to be due to the presence of large numbers of dislocations penetrating from the substrate to the surface in epitaxial crystals, the details of this are not fully understood. Moreover, in p-type GaN-based compound semiconductors, the resistivity is higher than the resistivity of n-type GaN-based compound semiconductors, there is hardly any horizontal spreading of current in the p layer in the case in which metal is simply deposited on the surface thereof, thereby in the case of adopting an LED structure having a pn junction, light is only emitted downward directly beneath the positive electrode.
Consequently, current diffusivity is enhanced by electron beam irradiation or high-temperature annealing to lower the resistivity of the p layer. However, the cost of a device for electron beam irradiation is extremely high, making it incompatible with production costs. In addition, it is difficult to uniformly process the wafer surface. In the case of high-temperature annealing as well, although it is necessary to employ a process at a temperature of 900° C. or higher in order to yield remarkable effects, there is the risk of decomposition of the crystal structure of the GaN and deterioration of reverse voltage characteristics caused by the elimination of nitrogen.
In addition, it has also been proposed to deposite Ni and Au at 10 nm each on the p layer for use as the positive electrode, and carry out alloying treatment in an oxygen atmosphere to promote lower resistance of the p layer as well as form a translucent and ohmic positive electrode (see, for example, Japanese Patent No. 2803742).
However, alloying treatment in an oxygen atmosphere causes the formation of an oxide layer on the surface of the exposed n-type GaN layer, and has an ohmic effect on the negative electrode. Moreover, Au/Ni electrodes that have been subjected to alloying treatment in an oxygen atmosphere have mesh structures, resulting in increased susceptibility to the occurrence of emission unevenness, reduced mechanical strength and the need to provide a protective film, which in turn leads to increased production costs. Moreover, since the Ni is heat treated in an oxygen atmosphere, when Ni oxides cover the surface and a pad electrode is formed on the translucent electrode, it has weak adhesive strength thereby preventing the obtaining of bonding strength.
In addition, it has also been proposed to form a Pt layer on the p layer for use as the positive electrode followed by heat treatment in an atmosphere including oxygen to simultaneously carry out resistance reduction and alloying treatment of the p layer (see, for example, Japanese Unexamined Patent Application, First Publication No. H11-186605).
However, since this method also involves heat treatment in an oxygen atmosphere, it has the same problems as those described above. Moreover, although the translucent electrode must be considerably thin (5 nm or less) to obtain a satisfactory translucent electrode with Pt alone, this results in an increase in the electrical resistance of the Pt layer. Thus, even if resistance of the Pt layer is reduced by heat treatment, current spreading is poor and emission of light is not uniform, leading to an increase in forward voltage (VF) as well as a decrease in emission intensity.